gpio: samsung: Update documentation

Update Samsung GPIO API documentation to reflect removal of
the s3c24xx specific gpio API. While at it, fix some typos.

The notes on conversion from s3c2410_* functions to the gpiolib
API are left here just in case there is any out of tree code that
still needs to be converted.

Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>